1. Field of the Invention
The present invention generally relates to underwater pelletizers and, more specifically, to an insulation plug mounted in and generally filling the recess in the die face of an extrusion die in an underwater pelletizer.
2. Description of the Prior Art
Known underwater pelletizers include an extrusion die having extrusion orifices extending therethrough, and through which molten polymer is extruded. A cutter hub with cutter knives is oriented in rotatable facing relationship to the face of the extrusion die to cut the strands of polymer into pellets. A water box with a water inlet and outlet provides circulation of water to cool and solidify the strands of extruded polymer to enable the knives on the cutter hub to cut the strands into pellets. The water circulating through the water box also entrains the pellets into a slurry of water and pellets which are discharged from the water box.
It is the present practice to provide a recess in the die face of the extrusion die and place insulation material in the recess to reduce heat transfer from the extrusion die and molten polymer being extruded through the extrusion orifices to the water circulating through the water box. The insulation material inserted into the recess is typically a gasket material mounted in place by a suitable mastic, glue or the like and covered with a thin metal plate in the recess. The thin plate is secured to the die by suitable fasteners to maintain the insulation material in place in the recess. The insulation material is subjected to substantial degradation due to the temperatures encountered in the recess and, in some instances, does not effectively insulate the central recess area of the die face. Ineffective insulation can result in excessive cooling of the molten polymer as it is being extruded through the extrusion orifices causing freeze off of the molten polymer at the die face.
The following U.S. patents, owned by the Assignee of this application, relate to underwater pelletizers and illustrate the relationship between the extrusion die, cutter hub and knives, water box with the water inlet and water and pellet slurry outlet:
In order to overcome the known problems associated with the insulation techniques of the prior art, the present invention provides an insulation plug in the form of a circular plate that substantially fills the recess in the die face of the extrusion die of an underwater pelletizer. The plate is of unitary construction and has a diameter substantially equal to the diameter, and a thickness substantially the same as the depth, of the recess in the die face. The circular plate reduces the transfer of heat from the molten polymer being extruded through the orifices in the extrusion die to the water circulating through the water box of the underwater pelletizer, but is not subject to deterioration and degradation as encountered with prior art insulation techniques.
The plate can be a solid one piece construction or a hollow two piece construction and should be made of a material having a low heat conductivity. Extrusion dies are typically made of carbon steel and the die body can be either solid or hollow (cavity type construction). Hollow die bodies can serve to provide a passageway for heating or cooling mediums, or simply for weight reduction or insulation purposes. The circular plate of the present invention is preferably made from stainless steel or low heat conductive ceramic materials, although other low heat conductive materials could be used. In the hollow plate construction, other high insulating materials could be placed in the hollow space.
The insulation plug formed by the circular plate of the present invention includes a raised boss around the periphery of the face (inner) of the plate adjacent the bottom of the recess in the extrusion die face. Hence, when the insulation plug is inserted into the extrusion die recess, a thin bottom air gap or space is formed between the adjacent insulation plug inner face and the bottom of the recess. As such, air is entrapped in this thin bottom air gap or space to enhance the insulating characteristics of the insulation plug with respect to heat transmitted from the bottom of the recess of the extrusion die.
The circular insulation plug of the present invention also includes a flange around the top of the cylindrical side wall of the circular plate to form a peripheral recess extending around the plate from the inner face of the plate (adjacent the bottom of the recess in the extrusion die) toward the opposite face (outer). The flange is sized to sealingly engage with the top inner peripheral surface of the recess in the extrusion die face. This sealing engagement of the flange with the periphery of the recess in the extrusion die and the peripheral recess in the side wall of the plate form a thin side air gap or space which entraps air between the side of the recess and the insulation plug and enhances the insulating characteristics of the insulation plug with respect to heat transfer from the side of the extrusion die recess. In addition, the peripheral flange around the periphery of the plate accommodates any variations in the dimensions of the recess in the extrusion die and the plate during their manufacture.
The insulation plug also includes a central opening with an inclined peripheral edge increasing in diameter toward the outer face of the insulation plug which is adjacent the cutter hub and cutter knives. The central opening in the plate receives an attaching or fastening bolt having a correspondingly configured bolt head. The bolt extends through the insulation plug and into a threaded bore in the extrusion die, or nose cone on the upstream side of the extrusion die, to secure the insulation plug in place in the recess in the die face of the extrusion die.
In certain embodiments of the present invention, the insulation plug includes a second raised boss on the inner face of the plate around the periphery of the attaching bolt opening which bears against the bottom surface of the recess to maintain the spacing for the bottom air gap against any compressive forces generated by the bolt attaching the insulation plug to the die plate or nose cone. In an alternate embodiment, an annular bushing or spacer can be placed in the countersunk bore of an existing die plate. The bottom, or one end, of the bushing bears against the bottom surface of the counterbore and the top, or other end, of the bushing bears against the inner face of the circular plate around the attaching bolt opening. The bushing thus serves to maintain the spacing for the bottom air gap against any compressive forces generated by the attaching bolt.
It is, therefore, an object of the present invention to provide a rigid insulation plug in the form of a circular plate for insertion into and retention in a correspondingly configured recess in the die face of an extrusion die of an underwater pelletizer.
A further object of the present invention is to provide an insulation plug in accordance with the preceding object and which will reduce heat transfer from the molten polymer and extrusion die of an underwater pelletizer to water circulating through the water box associated with the cutter hub, cutter knives and die face.
Another object of the present invention is to provide an heat insulation plug of low heat conductive material which can be solid or hollow and readily secured in the recess of the die face of the extrusion die of an underwater pelletizer for permanent insulation.
Yet a further object of the present invention is to provide a permanent heat insulation plug in accordance with the preceding objects in which the dimensional characteristics of the insulation plug are substantially the same as the dimensional characteristics of the recess to enable the insulation plug to closely fit into and substantially fill the recess in the die face of the extrusion die of an underwater pelletizer.
Yet another object of the present invention is to provide a heat insulation plug formed by a circular plate which includes a boss around the outside periphery of the inner surface, or face directed toward the bottom of the extrusion die recess, to form a narrow gap or space therebetween to entrap air between the adjacent faces of the recess bottom and insulation plug to enhance the insulating characteristics of the insulation plug.
A still further object of the present invention is to provide a heat insulation plug which includes a peripheral flange around the side wall top edge which is sized to sealingly engage with the top inner peripheral surface of the recess in the extrusion die to form a peripheral recess around the cylindrical lower side of the plug. This sealing engagement entraps air in the side recess to further enhance the insulating characteristics of the insulation plug. The peripheral recess also minimizes surface contact with the side wall of the extrusion die recess to accommodate for variations in the dimensions of the extrusion die recess.
Still another object of the present invention is to provide an insulation plug in accordance with the preceding objects in which the insulation plug is constructed of a low heat conductive material so as to improve the heat insulating properties of the insulation plug.
A still further object of the invention is to provide an insulation plug in accordance with the preceding objects in which the attaching bolt has a head that is recessed into the opening through the insulation plug in order to provide a substantially planar surface on the insulation plug, bolt head and die face.
One further object of the present invention is to provide an insulation plug in accordance with the preceding objects and which further includes structure around the attaching bolt opening on the inner face of the plate which prevents the central portion of the plate from deflecting inwardly when the attaching bolt is threaded into the die body or nose cone and maintains the spacing of the bottom air gap.
Yet another object of this invention to be specifically enumerated herein is to provide a heat insulation plug for the die face recess of an underwater pelletizer extrusion die in accordance with the preceding objects which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a device that will be economically feasible, long lasting and relatively trouble free in operation.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.